1. Field of the Invention
The present invention relates to a combined power plant.
This application is based on Japanese Patent Application No. 2007-088749, the content of which is incorporated herein by reference.
2. Description of Related Art
Recently, many thermal power plants that use liquid natural gas as fuel are in use. Combined power plants that include both gas turbine plants and steam turbine plants are generally used as such thermal power plants.
With these combined power plants, electric power is generated by driving a power generator using a gas turbine. Simultaneously, heat exhausted from the gas turbine (exhaust heat) generates steam by causing condensed water from a condenser in an exhaust-heat recovery boiler to vaporize. The generated steam is supplied to a steam turbine to drive the steam turbine. The power generator generates electric power by being driven by the steam turbine. The gas turbine and the steam turbine may be formed of different shafts to drive different power generators, or the gas turbine, the steam turbine, and the power generator may be connected along a single shaft to constitute one power generating unit.
It is known that the heat efficiency (power generation efficiency) of the above-described power plant can be increased by increasing the temperature of inlet combustion gas to the gas turbine. However, when the temperature of the inlet combustion gas is increased, high-temperature sections of the gas turbine (for example, the combustor, the stator blade, and the rotor blade) may experience damage or melting caused due to heat. Therefore, a technique for preventing damage or melting caused due to heat by cooling the high-temperature sections of the gas turbine has been proposed (refer to Publication of Japanese Patent No. 3068925 and Japanese Unexamined Patent Application, Publication No. 2000-248962).
Publication of Japanese Patent No. 3068925 and Japanese Unexamined Patent Application, Publication No. 2000-248962 disclose techniques involving using steam as a cooling medium for cooling the high-temperature sections of the gas turbine.
If the pipes used for supplying the steam (cooling steam system) are cool when steam is used as a cooling medium, drainage liquid, such as water drops, is generated due to condensation of part of the steam. The water drops may be guided together with the steam to the high-temperature sections of the gas turbine and may make contact therewith. The high-temperature sections of the gas turbine where the water drops make contact are locally cooled, and the durability may decrease due to thermal stress. Furthermore, since the high-temperature sections of the gas turbine have a complex cooling structure, there is a tendency to accumulate drainage liquid.
The cooling steam system is cold during start-up and shut down of the gas turbine plant.
There is a known method of preventing generation of water drops when the gas turbine is shut down by introducing dry gas to the cooling steam system and the gas turbine after the gas turbine is shut down so as to push out steam from the cooling steam system.
In many cases, however, because the temperature of the dry gas is lower than that of the steam, the temperature of the cooling steam system may decrease by introducing the dry gas thereto. Consequently, when the gas turbine plant is restarted, the pipes must be heated (warming must be performed) by passing steam through the cooling steam system again for a predetermined amount of time, thus causing a problem in that a long period of time is required for restarting.
In particular, there is a problem in that a long period of time is required for restarting the gas turbine plant after a relatively short shut down period, such as a weekly start and stop (WSS) and a daily start and stop (DSS).